/Engine/Plugins/Runtime/GeometryProcessing/Source/DynamicMesh/Public/Operations/WeldEdgeSequence.h
Checks for edges between terminating vertices of the input spans and attempts to delete them if possible by deleting the adjacent triangle. These cases would prevent MergeEdges() from succeeding because it won't implicitly collapse/delete triangles which connect the two edges being merged.
EWeldResult::OK on success
[EWeldResult](API\Plugins\DynamicMesh\Operations\FWeldEdgeSequence\EWeldResult) CheckForAndCollapseSideTriangles()